New study: Climate change shifts timing of floods in Europe

A team of 46 scientists analyzed five decades of data on river flooding in Europe, leading to the strongest evidence yet that climate change affects this important natural process.

In northeastern Europe, where rising temperatures have accelerated snowmelt, the team found earlier flooding at more than 80 percent of the data collection locations.

Across the rest of the continent, the impacts of climate change were less direct.

In some places, such as the Atlantic coast, the researchers found a shift toward earlier flooding. Some parts of Europe near the Mediterranean Sea experienced flooding later in the year.

When rivers flood, the results can be devastating, ruining crops, disrupting ecosystems and destroying homes. The UN estimates that we lose $104 billion a year to swelling rivers. Understanding how climate change alters flooding is critical, but until now, researchers haven’t been able to home in on discernible patterns over large geographic areas.

Recently, however, a team of 46 scientists took a new approach to this question, looking at when the floods occurred in the course of a year.

It led them to a qualified conclusion: “Climate change has impacted flood timing in Europe in the past 50 years, but it did so in different ways, in different parts of Europe,” said Gunter Blöschl, a hydrologist at the Vienna University of Technology, at a press conference. Blöschl is the lead author of a study published in the August 11 issue of the journal Science.

Climate scientists have long suspected that the effects of the warming on the Earth would show up in the flooding of rivers. They know that climate change impacts snowmelt, rainfall and soil moisture — important contributors to the flooding of waterways. But previous studies focused on measuring flood sizes, which can fluctuate as a result of deforestation, land use change, or the way humans alter the course of rivers. That has made it difficult to determine whether changes to river flooding could be attributed to climate change.

The timing of these floods is less susceptible to these factors. It is particularly sensitive to Europe’s seasons — leading the researchers to believe that it would be a good indicator of the effects of climate change. They pulled together measurements from more than 4,200 stations in eight countries going back to 1960, and they plotted out where and when floods occurred in that time span.

To be sure, the results bore out the complexity that river flooding is known for. But regionally, patterns began to take shape.

For example, in northeastern Europe, which includes much of Scandinavia and the Baltic states, river floods occurred earlier in the year at more than 80 percent of the measurement station locations.

“This is a very, very consistent pattern over such a large domain,” said Berit Arheimer, a coauthor and hydrologist at the Swedish Meteorological and Hydrological Institute, who was also at the press conference. At half of those stations, the shift was more than 8 days over the 50-year study period.

Arheimer explained that melting snow plays a prominent role in river flooding in this part of the continent.

“We also see a very clear rise in temperature for this domain, so the snow season is becoming shorter and the snow starts to melt earlier,” Arheimer said.

“It should also be mentioned that the trends we found in the timing of floods in northeastern Europe are very much in-line with the predictions of future climates from climate model projections across this region,” she added. “We can say that we already … see the climate change very clearly here.”

In other parts of Europe, the results were less straightforward, said coauthor Jamie Hannaford, a hydrologist at the Center for Ecology and Hydrology in the U.K. and coauthor of the study.

“Our study certainly provides evidence of climate-driven trends in northwest Europe,” Hannaford said during the press conference. “They are consistent with human-induced global warming, but it’s not possible to conclusively attribute these changes.”

In the more temperate parts of Europe, changes to the climate can fiddle with the mix of rainfall and the water content of the soil, as well as other factors, that influence river flooding.

In Atlantic regions from Portugal to England, the team found that flooding happened earlier in the year now compared to 50 years ago. About half of the stations recorded a shift of eight days or more, and at 25 percent of the stations, the shift was more than 36 days. Around the Mediterranean Sea, however, parts of Europe experienced flooding later in the year.

“I don’t think the changes are yet significant enough that they would have triggered major societal adaptations,” Hannaford said. But if the climate change continues to alter flood patterns, that could change.

“If flooding actually sort of goes into … the winter season, that could have an impact on the spring agricultural activities,” he added.